Experimental setup 1 Design considerations

The main goal of the research presented here was to investigate, up to which speed 2D and 3D SS-OCT imaging is possible with good image quality by simultaneously applying several techniques to increase the acquisition rate. The performance of several different setups is compared.

Two main strategies are applied to scale the OCT imaging speed: Firstly, we use an FDML laser, especially designed for ultra-high speed wavelength sweep operation. Here, an FDML laser is the light source of choice because FDML is a stationary laser operating regime [36,37] and so in most practical cases it provides no fundamental sweep speed limitation [9,29], high output power [38–40], low intensity noise [27], wide sweep range [41], and minor phase noise as well as operation in the 1050nm [13,22], the 1310nm [9,41] and the 1550nm [33,39] range for various types of OCT applications. Second, we use a multi-spot scanning approach with 4 spots on the sample to quadruple the OCT line rate. The concept of our setup is similar to the one described in [40]. It is based on the idea of the multi-beam approach used in the commercial system by Michelson Diagnostics Ltd. (e.g. see [42]), however with transversely separated spots on the sample, reducing thermal stress.

As the depth scan rate increases, the power on the sample has to be increased as well, to maintain high sensitivity. Assuming shot-noise limited detection at 1310nm with a photodiode response of 0.95A/W and a backcoupling efficiency of 75% in the sample arm, a scan rate of 1MHz requires a power of 4.5mW on the sample to achieve 100dB sensitivity. Speed and sensitivity are inversely proportional, so for 20MHz, 90mW would be required which,

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

depending on the scanning protocol, might be above the laser exposure safety limits. However, light-induced stress on the sample can be reduced by using multiple well-separated beams. According to ANSI and European laser exposure standards, on skin the incident power is averaged over a 3mm aperture. If the individual spots are separated by significantly more than 3mm, the sum of the laser power of two or more spots can be higher than the power for an individual spot. A similar approach might relax the power constraints to some extent for retinal imaging, too.

Aiming at very high speed, the major design considerations were:

1. An FDML laser is used as swept laser source with the advantage of good sweep speed scalability at low relative intensity noise.

2. A newly developed high speed sweep filter is driven at the highest possible frequency which still provides sufficient filter response.

3. The concept of buffering [10] is applied to further increase the sweep rate.

4. Two separate booster amplifiers make optimum use of the two outputs from the buffer stage, providing sufficient optical power on the sample.

5. Multiple well-separated spots on the sample increase the effective scan rate and reduce thermal stress on the sample caused by the laser spots.

6. A specially tailored 1GHz photo-receiver system is used for the optimum compromise between analog electronic bandwidth and responsivity / transimpedance gain. In contrast to spectrometer-based FD-OCT systems, the main cost and complexity of an SS-OCT setup is associated with the laser source itself rather than the detector. The detection using a balanced photo receiver and a fast ADC is easily duplicated [40,42] and allows the use of several distinct imaging spots, scanning different parts of the sample [40]. Buffering [10] quite naturally matches this multi-spot design, since external buffer stages [13] usually have two outputs: Instead of wasting half of the power, each of the two outputs can supply half the number of the spots.

Due to the high fringe frequencies, ADC sampling rates in the GS/s range are required. Especially for economic multi-channel operation, this requirement currently limits the data converter bit depth to 8 bit. However, in agreement with the results reported in [43], we also observe in our system that 8 bits provide enough dynamic range: In Fig. 1 we compare the resulting image quality when data sampled with a 12 bit ADC (400MS/s digitizer by GaGe Applied) is artificially bit-reduced during post-processing. The images were created with a 100kHz SS-OCT system. It is noteworthy that artificial bit reduction to 8 bit during post processing corresponds to an ADC with an effective number of bits (ENOB) of nearly 8 while real world 8 bit ADCs usually have an ENOB of 7.x (e.g. National Semiconductor’s 8 bit ADCs ADC08D1000 and ADC08B3000 have an ENOB of 7.4 and 7.1 at 1GS/s and 3GS/s, respectively). Hence, unlike [43], we include images at bit depths below 8 bit in our comparison to show that significant image degradation occurs at an ENOB <7 and real world 8 bit ADCs are suitable for OCT imaging. As a consequence of reduced bit depth, the preamplifier gain of the ADC has to be set more carefully than with 12 or 14bit ADCs so that little dynamic range is wasted.

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

resolution for individual images was lowered in software during post-processing. Real-world 8 bit ADCs at 1-3GS/s correspond to an image quality similar to the 7 bit image.

Modern digital real-time oscilloscopes commonly provide adequate bandwidth on up to 4 channels with 8 bit each, making the use of 4 imaging spots a natural choice. In our case, the 3D acquisition data size is limited to 64 million samples per spot by the 256Mb storage of the oscilloscope (DPO7104 from Tektronix).

For the spot separation, a value of 2mm is a good overall compromise: Laser safety regulations make use of a 3mm aperture, hence only 2 of the 4 spots account for exposure. The “native” size of a 3D data set is then 8mm along the spot separation axis supporting 330 independent scan steps for a spot size of 24µm on the sample. Using 2-4 sampling points per mode field diameter suggests roughly 600-1200 scans per frame. Alternatively, a smaller spot separation could be used in order to trade in increased light-induced stress for higher scan range flexibility. For example, with 0.5mm spacing along the slow scan axis, the “native” 3D cubes can have any even length (2, 4, 6, 8mm,…) along the slow axis. However, this also comes with increased post-processing effort as for 8mm, 16 individual 3D-sub-cubes would have to be merged.

2.2. Balancing sweep frequency and buffering multiplier

In FDML, the sweep frequency of the laser is limited by the wavelength tuning speed of the optical band pass filter, which, in our implementation is a piezo electric actuator (PZT) driven Fabry-Pérot filter. As the wavelength tuning speed of a sinusoidally driven filter is a product of amplitude and frequency, either of these parameters can be increased to achieve higher tuning speeds. Especially when increasing the drive amplitude of the filter, the sweep of interest fills only a fraction of the period and therefore the concept of buffering can be used to fill up the duty cycle to 100% with delayed copies of one sweep [10,13]. Depending on the type of PZT used in the Fabry-Pérot filter, there are usually several mechanical resonances where the filter can be driven over an optical bandwidth of typically 100nm which gives adequate resolution in OCT. We found that the roll off performance of the FDML laser is improved if the filter is driven at a higher resonance frequency and lower amplitude, probably because the shorter FDML cavity length introduces less dispersion and self phase modulation [27,37]. In contrast, the buffering factor, i.e. the number of sweep copies, seems to have no observable effect on OCT performance, at least up to 16x buffering. Buffer factors above 16x are normally unpractical since they demand very large filter tuning ranges.

To find the maximum sweep speed of the filter, the optical response of each resonance frequency of the filter has been checked, and the one which allowed for fastest wavelength tuning speed chosen. Next, the highest possible sweep range was determined experimentally and an adequate buffer stage (4x, 8x or 16x) was built.

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

2.3. FDML laser sources

Three different FDML laser sources, called F, B8 and B16, were built and compared. The first source, F (see Fig. 2), features a fiber-based Fabry-Perot filter (FFP-TF, LambdaQuest LLC - special “no-gel” version with reduced damping), driven at 257kHz, followed by a 4x buffer stage resulting in a sweep rate of 1.0MHz. The sweep rate was limited by the response and the thermal stress of the piezo crystal in the filter. The other two lasers, B8 and B16 (see Fig. 3), use a home-built semi-bulk optics Fabry-Perot filter (BFP-TF) driven at 325kHz, followed by an 8x and 16x buffer stage, respectively. The B8 laser has a sweep range of more than 100nm at 2.6MHz scan rate while B16 provides a scan rate of 5.2MHz but is limited to 80nm by the filter: Larger tuning ranges were prevented by mechanical contact of the filter facets. The BFP-TF simply consists of two 0.5” mirror mounts holding the fibers, and the cavity makes use of a glass plate with reflective coating. For a compilation of the various parameters, see Table 2, which also includes the scanning protocol and 2D/3D-data set sizes in imaging application.

Table 2. Key Parameters of the 3 Different Setupsa

F B8 B16

FDML resonance frequency 257 kHz 325 kHz 325 kHz

Buffer factor 4 8 16

Eff. sweep rate per spot 1.0 MHz 2.6 MHz 5.2 MHz

Sweep range 110 nm 100 nm 80 nm (filter limited)

Filter FSR and width (−3dB) 205 nm, 340 pm 330 nm, 380 pm 330 nm, 380 pm

Linewidth (FWHM) 80 pm 150 pm 240 pm

Laser output power 4 х 6.35 mW 8 х 2.3 mW 16 х 1.17 mW Booster output pwr (no mod.) 2 х 90 mW 2 х 100 mW 2 х 85 mW Power on sample 4 х 16 mW (WS) 4 х 30 mW 4 х 25 mW

Measured sensitivity (SNR = 1) 105 dB 104 dB 98 dB

Shot noise limit 105 dB 104 dB 100 dB

Analog bandwidth 450 MHz (LPF) 1 GHz 1 GHz

Acquisition sample rate 1.25 GS/s 2.5 GS/s 2.5 GS/s

R number (roll-off) 0.34 mm/dB 0.21 mm/dB 0.10 mm/dB

−20dB roll-off OCT depth 6 mm 4.8 mm 2.3 mm

RIN@1GHz (sweep average) 1.0% 0.7% 1.5%

Samples per depth scan 1200 950 475

Depth resolution in tissue 11 um (WS) 11 um 13 um

3D scan size (D х L х F х S) 1200 х 600 х 85 х 4 950 х 640 х 100 х 4 475 х 1280 х 100 х 4

3D scan duty cycle 50% 90% (bidir) 90% (bidir)

Max fringe frequency (20dB) (760 MHz) 1.25 GHz 1.1 GHz

2D depth scan line rate 4.1 MHz 10.4 MHz 20.8 MHz

Sustained 3D depth scan rate 2.1 MHz 9.4 MHz 18.7 MHz

Sust. 2D pixel rate (1 spot) 450 MHz 1.25 GHz 1.1 GHz

Sust. frame rate (4 spots) 3.3 kHz 14.6 kHz 14.6 kHz Sust. 3D voxel rate (4 spots) 900 MVoxels/s 4.5 GVoxels/s 4.0 GVoxels/s a_Laser

F applied booster current modulation to Welch shape (WS), so that no software apodizing was necessary. Lasers B8 and B16 used constant booster current and a Hann shaped window function for imaging; their measured resolution is specified without apodizing. “bidir” denotes bidirectional scanning. The 3D scan size is limited by oscilloscope storage (256Mb): D х L х F х S specifies number of depth samples х number of depth scan lines per frame х number of frames х number of spots. For imaging at 1.25GS/s, a separate low pass filter (LPF) was used, so although the 20dB roll-off frequency is 760MHz without the LPF, the detection bandwidth of 450MHz has to be used for pixel and voxel rate computations.

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

generator; PC: polarization controller; LDC: laser diode controller)

The basic setup of the FDML laser F is similar to the ones described previously [9,13,44,45]. To minimize polarization effects, laser F makes use of circulators in the cavity as well as in the buffer stage. We found that due to the highly polarization dependent gain of the booster semiconductor optical amplifiers (SOA, Covega type BOA-1132), reliable operation of all buffered sweeps over at least 100nm required the use of circulators and polarization controllers (PC) in each branch. In contrast to this, the faster lasers B8 and B16 and their buffer stages could be operated very well without circulators, probably because the shorter fiber spools reduced polarization effects.

Both setups include a 99/1 coupler between the laser and the buffer stages for wavelength monitoring on an optical spectrum analyzer (OSA). Further 99/1 couplers after each buffer SOA were integrated for sweep amplitude shaping [45] and monitoring of polarization controller adjustments.

The output coupler in the FDML laser cavity was placed after the SOA and not after the filter so that more power is available for the buffer stages and the booster SOAs. Although this design features two successive SOAs without filter in between, we found that it outperforms approaches where the output coupler is placed after the filter: The booster SOA does amplify ASE from the laser SOA, but the substantially higher input power saturates the booster and results in good suppression of booster ASE background. The overall ASE background therefore compares favorably to setups where the ASE-free post-filter laser output is boosted.

Fig. 3. FDML laser “B8” with bulk Fabry-Perot tunable filter (BFP-TF), followed by an 8x buffer stage with 2 booster SOAs. The laser “B16” differs merely by adding another buffer stage element with 39m fiber delay.

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

All filters were driven by a multichannel arbitrary waveform generator (AWG; TTi TGA12104) followed by a home-built high speed piezo driver. Special, home built high-speed laser diode controllers (LDC) modulated the SOA current to enable FDML lasing over a fraction of a filter cycle only, as required for buffering (Laser F: duty cycle 25%, driver WL- LDC10D from Wieserlabs (www.wieserlabs.com), 6MHz bandwidth; lasers B8 and B16: duty cycles 12.5% and 6.25%).

2.3 Multi-spot interferometer

The OCT sample is scanned with 4 separate laser spots to effectively quadruple the 3D acquisition speed. Figure 4 shows the 4-spot interferometer. Each of the two boosted outputs from the buffer stage supplies two spots. A fused fiber coupler splits the light from the buffer SOA into reference (20%) and sample arm (80%). The reference arm includes an adjustable common freespace delay of ~30cm matched to the free air path length of the sample arm for equal dispersion. Each of the spots has an individual adjustable delay (~2cm) to match the coherence gate among the spots. By slightly misaligning its freespace coupling, the reference arm power can be attenuated. The circulator in the reference arm serves two purposes: It compensates the dispersion introduced by the circulator in the sample arm and effectively reduces the mechanical length of the freespace common delay by a factor of 2. This interferometer design has the advantage that less power is wasted than by using a 50/50 coupler and an attenuator in the reference arm.

Fig. 4. 4-spot interferometer (CIR: circulator; BPD: balanced photo diode).

The fringe signal contrast is maximized for each spot using a polarization controller (PC). The signal is detected with a home built low-noise dual balanced InGaAs photoreceiver (BPD) with 1GHz bandwidth and 3300V/W trans-impedance gain (WL-BPD1GA from Wieserlabs (www.wieserlabs.com)). The photoreceiver is AC-coupled with a lower cutoff frequency slightly higher than the sweep repetition rate, in order to suppress background signal introduced by the chromatic imbalance of the 50/50 coupler in front of the detector. Fringe signals from an isolated reflection for setup B16 are shown in Fig. 6 (right). Especially for large reference arm power near the excess noise limit, this chromatic imbalance forms the major electrical signal contribution on the ADC and hence limits the achievable dynamic range. By setting the lower 3dB cutoff frequency to 2·fsweep, signal contributions at the sweep

repetition frequency are suppressed by 9dB. Hence, the first 2 depth resolution elements (which are normally unusable anyway) are traded in for typically 3-6dB more dynamic range. 2.4. Multi-spot scanner optics

Light from the 4 individual sample arms from the interferometer is focused on the sample at 4 different locations separated by 2mm along the slow axis of the galvanometer (galvo) mirrors of the beam scanner unit. Apart from aberrations in the objective, the axial focus plane is the same for all spots. This setup (see Fig. 5, left) allows the imaging system to simultaneously acquire 4 complete 2D B-frames along the fast galvo axis, with a spacing of 2mm.

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

For 3D imaging, the fixed 2mm distance requires that the 4 3D volumes of each spot cover an 8mm range along the slow axis. In practice, to help merging the data sets, we chose to scan slightly more than 2mm along the slow axis to obtain at least ~10% overlap between the 3D volumes acquired by the individual spots.

Each individual sample arm fiber is first collimated with a 19mm achromatic doublet. Prismatic mirrors for beam steering ensure all 4 beams cross each other right between the two galvo mirrors. This way, the required free aperture of the mirrors is just insignificantly larger than for a single collimated beam. A 50mm achromatic doublet forms the objective. It is placed 50mm from the galvo mirrors to achieve telecentric scanning.

The sample is located in the focus 50mm in front of the objective. The 50:19 lens ratio results in a theoretical spot size (mode field diameter) of 24µm on the sample. 2mm spot spacing is achieved by an angle of 2.3° between individual beams. This requires a distance >14cm between the galvo mirrors and the prismatic mirrors due to the collimated beam diameter of 5.5mm.

Since the achromatic doublet is not specially corrected for off-axis projection, the backcoupling efficiency for the outer beams (~40%) is reduced compared to the central beams (~60%). Furthermore, the axial focal position differs slightly which can be compensated by adjusting the individual collimators. Despite these differences, Fig. 6 (left) shows that OCT imaging performance of all 4 spots is similar.

Fig. 6. Left: Frames from the 3D data set shown in Fig. 12 (center) acquired with setup B8 at 2.6MHz depth scan rate. Each frame is from a different spot of the multi-spot setup and shows that all 4 spots deliver similar OCT performance. Each frame consists of 600 A-scans with 512 depth samples each; the 2D frame rate during 3D scanning was ~3.6kHz. Right: Interference fringes from the setup B16 acquired at 2.5GS/s as used for imaging. The upper graph shows 25 sweeps and the lower graph is a magnification of the same data set showing a single sweep.

#126756 - $15.00 USD Received 9 Apr 2010; revised 4 Jun 2010; accepted 15 Jun 2010; published 30 Jun 2010

Fig. 7. Left: En-face cut (605 х 360 pixel) at depth position 100 samples (0.7mm) showing human nailfold. The fast axis is oriented horizontally. Small images show magnifications of